Devices for confining ions are usually referred to as ion traps and generally utilize electric fields to confine (i.e., hold) ions within a specified region, although some types of ion traps utilize a combination of electric and magnetic fields to confine the ions. Most ion traps utilize specially-shaped electrodes to produce electric fields having shapes that are suitable for confining the ions. For example, a large majority of ion traps used hyperbolically-shaped electrodes to produce or generate quadrupole electric fields that are suitable for confining ions. Because the shape or configuration of the electric field in an ion trap is highly correlated with the shape of the electrodes used to establish the field, the shape of the electric field can be altered or changed by changing the configurations (e.g., shapes and relative spacings) of the electrodes of the ion trap.
The ability of a particular electric field to effectively trap or confine the ions depends on a large number of parameters, including the mass of the ions to be confined as well as the pressure within the ion trap. Therefore, if an ion trap is to function effectively, the ion-confining electric field produced by the ion trap must be tailored to the specific application. For example, an ion trap designed to operate in a high-vacuum environment, such as that associated with ion mass spectrometry, will not function effectively in a higher pressure environment, such as that typically associated with ion mobility spectrometry. Consequently, ion traps designed for use in ion mass spectrometers generally cannot be used in ion mobility spectrometers and vice-versa. Instead, the ion trap must be specifically designed for the particular application.
Devices for guiding ions are often referred to as ion guides and are often used to guide ions from an ion source to an ion trap. As was the case for ion traps, ion guides utilize electric fields to guide ions along a specified path or corridor, although ion guides utilizing a combination of electric and magnetic fields have also been used. A commonly used ion guide design utilizes several pairs of elongated rods or cylinders arranged around a central axis. An electric potential placed on opposed pairs of rods results in the formation of an electric field suitable for confining the ions to an area around the central axis. The ions can be made to move along the axis by imposing a suitable electric field gradient along the axis. As was the case for ion traps, the ability of a given ion guide to function effectively requires that the electric field produced thereby be tailored to the specific application. Therefore, ion guides suitable for use in high-vacuum environments are usually not suitable for use in high pressure applications, and vice-versa. That is, the ion guide must be specifically designed for the particular application.